The invention relates generally to a color or monochrome reprographic printing system, and more particularly concerns apparatus for delivering the paper or other media to a photoconductive surface, offset printing roll, etc.
In an electrophotographic printing machine, a photoconductive member (often a photoreceptor belt) is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is treated with toner particles and is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated in a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. Alternately, a plurality of images may be superimposed on the photoreceptor surface, and transferred simultaneously to the sheet. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
Surface irregularities in the paper may be occur as the paper (copy media) travels through the machine. Such irregularities may be caused by the various rollers and belts which carry the paper through the printing or other machine processes. For example, irregularities could occur as side one is fused before transferring an image to side two, or because of the characteristics of the paper stock due to manufacturing, humidity, etc. In addition, misalignment, excessive force, etc. may create localized deformities in the copy paper. As a result, air gaps may form between the paper and the photoreceptor belt. Such gaps result in poor transfer of toner from the belt to the paper, which may, in turn, cause deletions or distortions in the printed copy.
It is a purpose of this invention to improve the delivery of the paper to the photoreceptive belt and throughout the printing machine to minimize the formation of surface deformatities, such as wrinkles, etc. and thereby improve the engagement of the paper to the photoreceptive belt. It is another purpose of this invention to accomplish the above purpose without adding further media-contacting elements to the print media path. It is a further purpose of this invention to enable self-alignment of the handling means to accommodate media of various thicknesses and to provide means to apply uniform forces to sheets with deep texture.
U.S. Pat. No. 5,913,268, which issued in 1999 to the same assignee of this application, discloses a roller element for use in a copy machine. The disclosure of this patent is incorporated herein by reference. The roller described in the ""268 is a pneumatic roller having an internal chamber which is selectively connected to a source of fluid under pressure or a vacuum. Through holes in the exterior periphery of the roller, copy paper, which is being handled by the roller, may be selectively subjected to pressure or vacuum. This is said to improve the paper handling performance of the transport system. It is a purpose of this invention to adapt the pneumatic roller concept to enhance the delivery of copy paper to an image transfer station.
In the method and apparatus of this invention, a bearing surface of constant shape is constructed to provide a cushion of air between the print medium and the moving supporting surface. The air cushion causes a downward pressure to hold the medium to the transport element, i.e. a belt or drum, as the media is moved towards the image transfer station or other process station of the printing machine. The air bearing surface is positioned just upstream from the image transfer station to minimize distortion of the media as it is transferred to the electrostatic belt or drum.
To accomplish this, a housing is constructed just upstream of the image transfer station, adjacent the transport element. The housing contains a plenum for storing air under pressure. The housing has an exterior wall, the outer surface of which faces the path of the copy paper to form a bearing surface. The bearing surface is constructed to conform to the shape of the transport element. For example, a belt would generally require a flat bearing surface and a drum may require a slightly arcuate bearing surface.
The plenum is connected to a source of pressurized air through a series of valves. Sensors within the plenum monitor the pressure in the plenum and control the supply of air through the valves. An array of holes are constructed in the exterior wall to allow streams of pressurized air to escape to lubricate and form the bearing surface. The streams of pressurized air form a cushion outside the exterior wall to separate the copy paper from the wall and generate an overall pressure against the paper to encourage its flat engagement with the transport element. A flat, well aligned, presentation of the copy paper to the image transfer station may then be accomplished without supplementary transport elements which rely on physical contact.